First-principles study of the formation and migration of native defects in LiNH2BH3

Abstract

First-principles calculations based on density functional theory were carried out to investigate the formation and migration of native defects in LiNH₂BH₃. The structural properties and formation energies of H, Li, N and B related defects in various charge states were examined. Our analysis showed that the dominant atomic H defects are positively and negatively charged H interstitial (I_H⁺ and I_H⁻). The Li related defects are dominated by positively charged Li interstitial (I_Li⁺) and negatively charged Li vacancy (V_Li⁻). For N related defects, the energetically favorable defects are positively and negatively charged NH₂ interstitial (I_NH2⁺ and I_NH2⁻), while B related defect is dominated by neutral BH₃ interstitial (I_BH3⁰). Further results indicated that the neutral H₂ interstitial (I_H2) has the lowest formation energy (0.31 eV), suggesting that the major defect in LiNH₂BH₃ is I_H2. Investigation of migration processes of the defects showed that the migration barriers for V_(B)H⁺ (positively charged H vacancy on a B–H site), I_H⁺ and I_H⁻ are relatively high (0.50–0.68 eV), whereas moderate diffusion barriers are presented for V_(N)H⁻ (negatively charged H vacancy on a N–H site) and I_Li⁺ (0.29 and 0.32 eV, respectively). The V_Li⁻ and I_H2 defects can migrate with low energy barriers of 0.13 and 0.16 eV, respectively. With a low activation energy of 0.47 eV, I_H2 is the major diffusive species in LiNH₂BH₃. Our calculation results further suggest that the creation of the V_(N)H⁻, I_Li⁺ and V_Li⁻ defects is the rate-limiting step for their transportation in LiNH₂BH₃.